diff --git a/lib/index.d.ts b/lib/index.d.ts
index 764c435..dbc060f 100644
--- a/lib/index.d.ts
+++ b/lib/index.d.ts
@@ -85,7 +85,7 @@ declare namespace postcssValueParser {
      * @param node The node to stringify
      * @returns The serialized CSS representation of the node
      */
-    (nodes: Node): string;
+    (nodes: Node): string | undefined;
   }
 
   interface WalkCallback {
@@ -95,7 +95,7 @@ declare namespace postcssValueParser {
      * @param nodes The series of parsed nodes
      * @returns Returning `false` will prevent traversal of descendant nodes (only applies if `bubble` was set to `true` in the `walk()` call)
      */
-    (node: Node, index: number, nodes: Node[]): void | false;
+    (node: Node, index: number, nodes: Node[]): void | boolean;
   }
 
   /**